1. Field of the Invention
The present invention relates to a semiconductor charge pump, and more particularly to a Dickson type voltage increasing charge pump using MOS transistors for a current rectifier element.
2. Description of the Related Art
In recent years, there has been prevalent a nonvolatile semiconductor memory device featured in that even if power is down, no information is lost. In particular, there is a high demand for such a nonvolatile semiconductor memory device in management of information relating to redundancy of memory devices or maintaining chip specific information, a trimming process of an analog circuit, a speed matching process of a high speed computation logic or the like. Common requests for the nonvolatile semiconductor memory device in these uses include that a memory capacity may be about several thousands bits at most (may be small in size); that there is no need for rewriting information as long as it has been written once; that packaging on chip is inexpensive or the like. Namely, there has been a request for “inexpensively packaging a small-scale nonvolatile semiconductor memory device on the same chip”.
An irreversible nonvolatile semiconductor memory device using a fuse element for a memory element meets such a request. However, the conventional fuse element has been formed so as to thermally weld a wiring layer by means of laser irradiation. Therefore, the nonvolatile semiconductor memory device using a fuse element of this type has a disadvantage that information cannot be written after chip sealing (packaging) has been done.
In contrast, recently, a so called gate oxide film destruction type anti-fuse element has been used as a memory element for a nonvolatile semiconductor memory device formed to apply a high voltage to a gate oxide film of a MOS transistor and utilize a breakdown phenomenon of an oxidization insulation film. In the case of this irreversible nonvolatile semiconductor memory device using the anti-fuse element, it is possible to write information after sealing.
On the other hand, in terms of the nonvolatile semiconductor memory device, there is well known a nonvolatile semiconductor memory device capable of electrically erasing information. A nonvolatile semiconductor memory device of this type includes: a Flash EPROM (Electrically Programmable Read Only Memory); an MRAM (Magnetic Random Access Memory); and a FeRAM (Ferroelectric Random Access Memory). In the case of the nonvolatile semiconductor memory device of this type, specific memory elements are used, respectively, in order to package a number of large-scale memory elements in a small area. However, a dedicated manufacturing process is necessary for forming specific memory elements. The use of the dedicated manufacturing process increases a manufacturing cost. In addition, the use of the dedicated manufacturing process causes the following problems. For example, the above use of the process causes degradation of memory characteristics of other memory elements packaged, respectively, on the same chip; increases characteristic degradation or characteristic variation of an analog circuit element; and causes speed characteristic degradation of transistors for a high speed computation logic. Therefore, the request for “inexpensively packaging a small-scale nonvolatile semiconductor memory device on the same chip” is not satisfied in the case of such a nonvolatile semiconductor memory device capable of electrically erasing information.
A specific manufacturing process is not required to form a memory element in the case of the above-described irreversible nonvolatile semiconductor memory device using a gate oxide film destruction type anti-fuse element. Thus, this memory device is suitable for a request for “inexpensively packaging a small-scale nonvolatile semiconductor memory device on the same chip”. However, an information write operation requires a high voltage. Therefore, in order to ensure packaging on the same chip, it is indispensable to achieve means for supplying a high voltage.
The simplest high voltage supply means can include a configuration such that high voltage power is supplied from the outside of the chip via an external supply pin. With this configuration, there is a danger that electrostatic breakdown of a fuse element occurs due to electrostatic application to the external supply pin. Thus, it is necessary to additionally provide a protective element to the external supply pin for protecting the fuse element from electrostatic breakdown. However, this configuration must meet a contradictory specification such that it permits high voltage application for storing information in the fuse element, whereas it prevents high voltage (electrostatic) application that causes electrostatic breakdown of the fuse element. Therefore, it is not permissible to additionally provide the protective element to the external supply pin. This makes it impossible to utilize an advantage of a gate oxide film destruction type anti-fuse element that information can be written after sealing.
Another means for supplying a high voltage can include a configuration such that a voltage increase power source such as a Dickson type charge pump is packaged on the same chip (for the Dickson type charge pump, reference should be made to J. F. Dickson, “On-chip high-voltage generation in MNOS integrated circuits using an improved voltage multiplier technique,” IEEE J. Solid-State Circuits, vol. SC-11, pp. 374-378, June 1976.) However, specific elements such as diodes and high withstand voltage transistors are required to configure the Dickson type charge pump. However, requiring such specific elements reduces an attraction attained by using a gate oxide film destruction type anti-fuse element that does not require a specific manufacturing process for a nonvolatile semiconductor memory device.
For example, in the Dickson type charge pump in which five current rectifier elements have been connected in series, a diode having a dual well structure is indispensable as a current rectifier element in order to normally operate this charge pump. This is because, if a parasitic diode is used for the current rectifier element that configures the Dickson type charge pump, when a PN junction of the parasitic diode is biased in a forward direction, for example, a PNP parasitic bipolar composed of a P+ type diffusion layer being an anode terminal, an N-type well being a cathode terminal, and a P-type substrate becomes electrically conductive. Therefore, a current charged from the anode terminal leaks onto the P-type substrate, and the charge pump does not function normally. In contrast, if a diode having a dual well structure is used for the current rectifier element that configures the Dickson type charge pump, all electrons being minority carriers charged from an N+ type diffusion layer serving as a cathode terminal are collected to the anode terminal without any leak to another node. Therefore, the charge pump functions normally. However, the diode having the dual well structure is high in its manufacturing cost, as compared with that of the parasitic diode. In this case, even if there has been used a gate oxide film destruction type anti-fuse element that does not require a specific manufacturing process, an inexpensive nonvolatile semiconductor memory device cannot be provided.
In the Dickson type charge pump, there has been reported a configuration in which diode-connected N-channel MOS transistors are used as current rectifier elements, and these elements are connected in series. (For example, reference should be made to Toru Tanzawa and Tomoharu Tanaka, “A Dynamic Analysis of the Dickson Charge Pump Circuit”, IEEE Journal of solid-state circuits, vol. 32, No. 8, August 1997, pp. 1231-1240). However, in the case of this Dickson type charge pump, a back gate effect of rectification characteristics of the diode-connected N-channel MOS transistors becomes a problem. The back gate effect used here denotes that a voltage (threshold voltage), at which a current starts flowing between a source and a drain, becomes high in accordance with the increase in back gate voltage. In the case where an N-channel MOS transistor having such rectification characteristics has been used as a current rectifier element, a current drive capability is lowered as a voltage-increase voltage of the Dickson type charge pump becomes high, and finally, no current flows. Thus, even in the Dickson type charge pump composed of diode-connected N-channel MOS transistors, it has been impossible to provide an inexpensive nonvolatile semiconductor memory device using a gate oxide film destruction type anti-fuse element.